stream.ffmpegmux: show warning if unavailable

src/streamlink/stream/ffmpegmux.py

@@ -90,12 +90,16 @@ class FFMPEGMuxer(StreamIO):
     @lru_cache(maxsize=128)
     def resolve_command(cls, command: Optional[str] = None) -> Optional[str]:
         if command:
-            return which(command)
-        resolved = None
-        for cmd in cls.__commands__:
-            resolved = which(cmd)
-            if resolved:
-                break
+            resolved = which(command)
+        else:
+            resolved = None
+            for cmd in cls.__commands__:
+                resolved = which(cmd)
+                if resolved:
+                    break
+        if not resolved:
+            log.warning("FFmpeg was not found. See the --ffmpeg-ffmpeg option.")
+            log.warning("Muxing streams is unsupported! Only a subset of the available streams can be returned!")
         return resolved
 
     @staticmethod

tests/stream/test_ffmpegmux.py

@@ -52,6 +52,23 @@ class TestCommand:
         with patch("streamlink.stream.ffmpegmux.which", return_value=resolved):
             assert FFMPEGMuxer.is_usable(session) is expected
 
+    def test_log(self, session: Streamlink):
+        with patch("streamlink.stream.ffmpegmux.log") as mock_log, \
+             patch("streamlink.stream.ffmpegmux.which", return_value=None):
+            assert not FFMPEGMuxer.is_usable(session)
+            assert mock_log.warning.call_args_list == [
+                call("FFmpeg was not found. See the --ffmpeg-ffmpeg option."),
+                call("Muxing streams is unsupported! Only a subset of the available streams can be returned!"),
+            ]
+            assert not FFMPEGMuxer.is_usable(session)
+            assert len(mock_log.warning.call_args_list) == 2
+
+    def test_no_log(self, session: Streamlink):
+        with patch("streamlink.stream.ffmpegmux.log") as mock_log, \
+             patch("streamlink.stream.ffmpegmux.which", return_value="foo"):
+            assert FFMPEGMuxer.is_usable(session)
+            assert not mock_log.warning.call_args_list
+
 
 class TestOpen:
     FFMPEG_ARGS_DEFAULT_BASE = ["-nostats", "-y"]